Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over packet-switched communications networks and, more specifically, to methods and apparatus for precision time transfer where the inherent packet delay variation and possible asymmetry introduced in networks is avoided or mitigated.
Description of the Related Art
It has been recognized that synchronizing network elements in communications networks to a high level of precision enables the provision of advanced services. In fact, time and frequency alignment is essential to certain types of systems. For example, accurate time alignment is required by cellular networks, services carried out in real time over a network, and network elements that use packet-based signal formats for multiplexing, transmission, and switching. Similarly, frequency alignment is required in time-division multiplexing (TDM) and media streaming systems that require fixed video or audio sample rates across multiple clients. Modern mobility services are demanding more and more bandwidth and lower and lower latency, placing increasing demands for tight time coordination in the wireless transport (radio access networks). Another example is the demand for much higher time coordination of transactions in high speed financial trading. To gain some perspective into the increasing demand on precision, whereas legacy time distribution targets at the more stringent end were on the order of sub-milliseconds, emerging application and services targets are now on the order of sub-microseconds.
The distribution of time over packet networks is now common in the industry. The dominant method is the use of the Network Timing Protocol (NTP) for support of general timing applications in general computing applications. However, these implementations, based on existing standards and conventions are only suitable for time alignments on the order of (several) milliseconds. Over the last decade, a new protocol, Precision Timing Protocol (PTP) has emerged supported by industry standards (IEEE 1588-2008, ITU-T Recommendations in the G.827x series). The key differentiator between NTP and PTP is that new levels of precision that can be obtained with PTP support the needs of a variety of new applications and services. Both PTP and NTP are protocols for exchanging time-stamps associated with time-of-arrival and time-of-departure of designated packets and are thus, in principle if not practice, capable of similar performance levels.
Packet-based network synchronization methods such as PTP and NTP transfer time and frequency references using packets containing time stamps that identify the times of departure/arrival of packets. These protocols can be used to distribute timing and frequency alignment throughout a network in a point-to-point fashion similar to the way that TDM networks distribute frequency alignment. For specificity the discussion here considers PTP though essentially identical statements apply in the case of NTP and all other packet-based time transfer methods.
A PTP “grandmaster”, which is the root timing reference, can transfer time to a network element coupled to it, a “slave” instantiated on that network element can reconstitute the time, and then a “master” connected to the slave in the same network element can transfer time to a subsequent network element in this point-by-point fashion. A device that has both slave and master functions and which can be used to transfer time in a relay fashion is referred to as a “boundary clock.” If every network element between the “grandmaster” and the end-point “slave” clock is a boundary clock, then packet delay variation (PDV) introduced by the network is moot since the timing flow is point-to-point. This architecture is referred to as “full on-path support” or “full protocol-level timing support” from the network. However, even where full on-path support is provided, the asymmetry in the connecting links remains as a dominant source of time transfer inaccuracy.
PTP can thus operate in a mode where the “slave” clock in a network element can communicate directly with the “master” clock system for timing purposes and that network element is referred to as a boundary clock, as described above (the same principle applies in NTP though the terminology is different). The case where the network does have intervening network elements that do not provide timing support between the end-point master (i.e., the last master in a chain, which could be the grandmaster in the case where a PTP-unaware network is between the grandmaster and the slave, or otherwise the last boundary clock between the grandmaster and the slave) and slave devices can be viewed as an architecture that provides “partial timing support” from the network. In all cases, the accuracy of two-way time-transfer protocols is adversely affected by asymmetry introduced by the communications network connecting the two network elements, including asymmetry in the physical medium, asymmetry in the construction of the forward and reverse paths in the network elements, and other sources. PTP and NTP assume that transit delays between master and slave clocks are symmetric, i.e., the transfer packet delay from a master clock to a slave clock is equal to the transfer packet delay from the slave clock to the master clock. But because forward and reverse physical paths are often different in coupled network elements, they are typically not symmetric. This is a source of asymmetry.
In all packet-based timing transfer methods, including PTP and NTP, there are two principal sources leading to synchronization error. One source of time error that cannot be corrected by the protocol is asymmetry, discussed above; the other is transit PDV. Asymmetry as considered here is the difference in transit delay of the designated event packets in the two directions between the communicating clocks. PDV, on the other hand, is an expected phenomenon in packet networks and contributes to asymmetry as well as to dynamic time alignment error. Whereas PDV is dependent on network loading, there is an underlying asymmetry component that is entirely independent of the network loading and depends, as previously noted, on the path between the clocks. The path, as considered here, includes all transmission links, including multiplexers and signal-format converters and transmission media, and intermediate network elements, such as switches and routers, between the communicating clocks. Clearly the impairments introduced by PDV and asymmetry generally increase with increasing number of network elements in the path as well as path geographical length.
Prior art teachings for methods to reduce the deleterious impact of PDV in packet networks include the use of boundary clocks and transparent clocks. However, such methods do not address the deleterious impact of asymmetry in the physical plant (i.e. cables and physical layer transmission). There is a need, therefore, for a system that addresses PDV and asymmetry.